Plating is the process of electrochemically depositing the layer onto a surface of a workpiece. In a typical plating process according to the prior art, a positively-charged element, the anode, is disposed in a plating fluid. A negatively-charged workpiece is also immersed in the fluid. The electric charge between the anode and the cathode creates ions in the plating fluid. These ions are then electrically attracted to the workpiece and are deposited on the surface.
The rate of ionic exchange at the surface of the workpiece can affect the quality of the plating. An increased ionic exchange rate can produce an improved plating grain structure. In addition, such increased ionic exchange rate promotes higher current densities. This results in faster plating and, therefore, a higher plating throughput.
A high ionic exchange rate can be promoted by continually refreshing the plating fluid at the surface of the workpiece. For example, a laminar fluid flow can be created by moving the plating fluid across the surface of the workpiece. However, a laminar fluid flow is relatively slow. The plating fluid is subject to the effects of friction at the surface of the workpiece. As this frictional force is increased, the plating fluid is slowed. At the molecular level, the plating fluid flow can be stopped. The ionic exchange rate is therefore decreased, and the plating process slowed. Thus, the ionic exchange rate produced by a laminar flow is limited.
It is well-known to use a turbulent plating fluid flow to provide a high ionic exchange rate. However, more energy is required to generate a turbulent fluid flow than a laminar flow. In addition, it is difficult to produce a uniform ionic exchange rate at each point on the surface by using a turbulent flow. Thus, a non-uniform coating will be formed over the surface. The maximum ionic exchange rate is therefore limited by the maximum amount of turbulent flow that permits the creation of a relatively uniform coating.
One prior art method for increasing turbulent flow is by circulating the plating fluid in the plating tank. FIG. 1 is a top plan view of a dip tank plating system according to the prior art. In the Figure, three parallel rows 12, 14, 16 of in-line anode baskets 18 are disposed in a plating tank 10. The plating tank holds a plating fluid (not shown). A cathodic workpiece 20, 22 is immersed in the plating fluid, between the rows of anode baskets.
Spargers 24, 26, 28 are located, for example, at the bottom of the tank, such that spargers are positioned on both sides of the workpiece. The spargers release air bubbles to agitate the plating fluid. The resulting agitation can improve the plating efficiency of the system. However, one known problem with such system is that the air bubbles lower the density of the plating fluid. Each air bubble displaces the conductive plating fluid with an insulative air bubble. Furthermore, air bubbles can also increase the evaporation of the plating fluid. Thus, the rate and amount of air bubbles introduced into the tank must be balanced by the lowered density of plating fluid caused thereby
Another problem inherent to the dip tank system is that air bubbles can adhere to the surface of the workpiece during plating. An adhering air bubble can then detach from the surface, leaving a recessed portion in the plated surface of the workpiece. To produce a consistent, and even plating, it is important to constantly detach adhering air bubbles from the workpiece surface. The maximum plating efficiency of the prior art dip tank system is therefore limited by the ability of the system to detach adhering bubbles from the workpiece surface. Under ideal conditions, the prior dip tank can achieve a plating current density of approximately 10-150 amperes per square foot, with a typical plating current density of between 10-30 amperes per square foot.
The circulation plating system attempts to solve these recognized problems of dip tank plating systems. FIG. 2 is a side sectional view of a circulation plating system 38 according to the prior art. Such circulation plating systems include the SER-DUCTOR.TM. Systems developed by Serfilco Ltd. of Northbrook, Ill.
In FIG. 2, a centrifugal pump (not shown) draws plating fluid 36 from a plating tank 34 and delivers this plating fluid back into the tank through a plurality of nozzles 32. The plating fluid is thereby circulated within the plating tank.
However, one problem with a circulation plating system is achieving a constant circulation of plating fluid directed at all locations on a surface 31 of the workpiece 30. Differing rates of circulation result in different ionic exchange rates across the surface, producing an uneven coating. For example, the plating fluid circulation 35 dispersed by the different nozzles could result in locations on the surface at which the ionic density is significantly greater, or significantly less than other locations. This is a significant disadvantage in plating devices that require extreme precision.
In the Serfilco system, the nozzles are generally not directed at the surface of the workpiece. Directing an inadequate amount of nozzles at the workpiece surface promotes an unequal distribution of ions at the surface. Thus, the plating current density is limited by the circulation rate which can be achieved by nozzles directed away from the workpiece surface. The Serfilco circulation plating system can achieve plating current densities that are as high as 2 times, and typically from 1.25 to 1.5 times greater than those achieved using a dip tank system.
The use of a plating fluid flow to achieve a higher ionic exchange rate is also known in the prior art. An example of such flow process is the fountain plating process of the International Business Machines Corporation (IBM) of Armonk, N.Y. FIG. 3 is a side view of a portion of a fountain plating apparatus 44 according to the prior art.
In the fountain plating process, a vertical nozzle 46 directs a fountain of plating fluid 48 up towards the rotating workpiece 50. The plating fluid contacts the surface 52 of the workpiece at a velocity sufficient to promote an increased ionic exchange. A plurality (not shown) of these fountains are used in the fountain plating system.
However, the vertical fluid stream used in the fountain plating process is subject to the effects of gravity. Gravity attracts the fluid stream, pulling the fluid downwards. Thus, the stream curves as it approaches the surface of the workpiece. This curvature of the fluid stream can result in a "dead spot" 54 at which there is a reduced fluid flow contacting the surface. The resulting unequal ionic distribution at the surface produces an uneven plating. The workpiece is rotated over the fountains to compensate for the uneven ionic distribution produced by the fluid streams. This procedure requires the additional use of a motor and a control system for the workpiece rotation.
Unfortunately, practical and effective techniques for plating particulate materials are not readily available. Such particulate materials include the particle interconnect material described in DiFrancesco, Method For Cold Bonding, U.S. Pat. No. 4,804,132 and DiFrancesco, Particle-Enhanced Joining of Metal Surfaces, U.S. Pat. No. 5,083,697. Particle interconnect material contains coated metal particles, which are formed of diamond, silicon carbide particles coated by metals such as nickel or copper. These particles range in size from approximately 3 .mu.m to approximately 200 .mu.m. Particle interconnect material is typically used to pattern regions of thermal, electrical, and mechanical conductivity or insulation.
The particles can be dispersed in a binder, such as an adhesive, as described in DiFrancesco, Patternable Particle Filled Adhesive Matrix for Forming Patterned Structures Between Joined Surfaces, U.S. Pat. No. 5,670,251. Whether a particle settles or remains suspended in a plating fluid depends upon interdependent factors such as particle size, particle shape, and the relative density of the particle in a given density fluid.
Another significant factor is the fluid velocity local to each individual particle and to each location in the tank. Insufficient fluid velocity can cause particles to settle to the bottom rather than to remain suspended in the plating fluid. This frequently occurs along the sides of the plating tank. Faster particle settling generally occurs in slower, horizontally-flowing fluids, and for spherical and cylindrical particles. Larger particles, typically those in excess of 15 .mu.m, tend to sink in the plating material. The smaller particles tend to remain in suspension in the plating fluid. This can be a significant problem in circulation plating systems.
The gas bubbles used in the prior art dip plating systems do not provide sufficient turbulence to support the larger particles in the fluid. Furthermore, while smaller particles can attach to the air bubbles and remain suspended, the larger particles cannot be so supported and settle to the bottom of the plating tank. In addition, the air bubbles reduce the density of the plating fluid, as previously discussed.
Similar problems are inherent to the fountain plating system. The fountain plating flow must achieve sufficient velocity to support the particles. However, the particle impact caused by a high velocity flow can result in surface damage.
The prior art plating systems are also not readily adapted to produce patterned plating. For example, when patterned over a surface, the previously described particle interconnect material can provide areas of electroconductivity and insulation. However, the ion deposition pattern is not readily controlled using the methods known in the prior art.
It would therefore be advantageous to provide a plating method and apparatus that increases the ionic exchange rate at the surface of a workpiece. It would be a further advantage if such method and apparatus provided a uniform deposition of plating materials over the workpiece surface. It would be yet another advantage if such method and apparatus were operable to deposit particulate materials such as particle interconnect. Additionally, it would be an advantage if such method and apparatus permitted the patterning of plated material on a workpiece surface.